Quote Originally Posted by Tellos Athenaios View Post
Easy: because a=b, a^2 - ab = 0. Therefore you cannot cancel the factor (a^2 - ab) because that amounts to division by zero which is undefined. Or in other words in step 7 there is written in a convoluted manner that 2 * 0 = 1 * 0. First try, full marks!
acid -> face